I used one of the blow up aquarium ones from ToyrUs, it was on sale for like $10 or something. Plenty of room and the big padded walls were nice, deep enough too. Unfortunately we left ours outside, so I'll probably go get another one.

The instructions they gave were to cover pillows with garbage bags and duct tape it closed for padding in the tub. Then put a 10x25 3-5ml plastic painters drop cloth over it. It's roomy and DEEP which is the best part.

I know it's expensive to buy one, but I wonder what the resale would be like...

I thought I'd point out that with the fish blow up pool, although it's kinda funny having a bunch of fish around while you are laboring, it was really, really nice laying my head down on the side of the pool, since it was inflated it felt like a pillow and I could drape my arms over the side comfortably. This was the point at which I almost fell asleep in between contractions. The floor was padded too.
Just wanted to point that out. You may not know up until you labor what position will be the most comfortable, whether leaning over on your knees or sitting leaning against the side of the pool or even just on your hands and knees.
I hope it goes well for you!!